Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. Such elements are described in, for example, Research Disclosure, June 1978, Item No.17029 and U.S. Pat. Nos. 3,080,254, 3,457,075 and 3,933,508.
The aforesaid thermally processable imaging elements are often provided with at least one protective layer. The protective layer can be a overcoat layer or a backing, or the element may have both a protective overcoat layer and a protective backing layer. The overcoat layer is an outer layer on the side of the support on which the imaging layer is coated and the backing layer is an outer layer on the opposite side of the support. Generally these layers are the outermost layers of the element. Other layers which are advantageously incorporated in thermally processable imaging elements include subbing layers and barrier layers.
To be fully acceptable, a protective layer for such imaging elements should: (a) provide resistance to deformation of the layers of the element during thermal processing, (b) prevent or reduce loss of volatile components in the element during thermal processing, (c) reduce or prevent transfer of essential imaging components from one or more of the layers of the element into the overcoat layer during manufacture of the element or during storage of the element prior to imaging and thermal processing, (d) enable satisfactory adhesion of the protective layer to a contiguous layer of the element, (e) be free from cracking and undesired marking, such as abrasion marking, during manufacture, storage, and processing of the element, (f) provide adequate conveyance characteristics during manufacture and processing of the element, (g) not allow blocking, ferrotyping adhering or slippage of the element during manufacture, storage, or processing and (h) not induce undesirable sensitometric effects in the element during manufacture, storage or processing.
A protective layer also serves several important functions which improve the overall performance of thermally processable imaging elements. For example, the protective layer serves to improve conveyance, reduce static electricity, reduce dirt and eliminate formation of Newton Rings.
A typical protective layer for thermally processable imaging elements comprises poly(silicic acid) as described in U.S. Pat. Nos. 4,741,992, 4,828,971, 5,310,640 and 5,547,821. Advantageously, water-soluble hydroxyl containing monomers or polymers are incorporated in the protective layer together with the poly (silicic acid). Other hydrophilic and hydrophobic protective layers are also known. These include those formed from poly(methyl methacrylate), cellulose acetate, crosslinked polyvinyl alcohol, terpolymers of acrylonitrile, vinylidene chloride, and 2-(methacryloyloxy)ethyltrimethylammonium methosulfate, crosslinked gelatin, polyesters and polyurethanes.
With photothermographic elements, it is usually necessary to produce a "duplicate image" of that on the imaging element for low cost dissemination of the image. The duplication process is typically a "contact printing" process where intimate contact between the photothermographic imaging element and the duplication imaging element is essential. Successful duplication of either continuous rolls or cut sheets is dependent on adequate conveyance of the imaging element through the duplication equipment without the occurrence of slippage or sticking of the protective overcoat layer of the photothermographic imaging element in relation to any of (1) the duplication equipment, (2) the duplication imaging element or (3) the backing layer of subsequent portions of the photothermographic imaging element (adjacent convolutions of the photothermographic imaging element if in a continuous roll or adjacent "cut sheets" in a stacking configuration). The latter of these phenomena is often referred to as "blocking".
The addition of matte particles to either or both protective layers of a thermally processable imaging element is commonly used to prevent adhering or "blocking" between the protective overcoat layer and adjacent backing layer with which it is in intimate contact during manufacture, storage, processing and photoduplication. Furthermore, the matte particles are desirable to impart desired frictional characteristics to the protective layers to achieve proper conveyance without sticking, blocking or slippage during the duplication process. The amount and particle size of the matte must be controlled as the wrong particle size and/or amount can cause conveyance, duplicate image quality and vacuum draw down problems. Another problem associated with the use of matte particles in protective layers of thermally processable imaging elements is dusting that comes from inadequate adhesion between the matte particles and the binder. In particular, larger matte particles are required to improve film roughness, but larger matte particles are more easily dislodged from the protective overcoat layer. The dislodged, or dusted, matte is can no longer provide the desired film roughness and it accumulates on the film or equipment surfaces causing various defects such as scratches, visible spots etc.
The properties of mattes are very important to their incorporation into film products. The matte improves or tailors the transport and vacuum smoothness properties of the final film product and can also provide increased protection from ferrotyping and blocking of the raw and processed film. The glass transition temperature (Tg) and composition of the matte determines the effect of processing conditions on the final matte properties, i.e. swellability, size, surface roughness, etc.
Three very important properties of a matte that determines whether it is best suited for use in a particular product application are:
1. particle size and size distribution PA1 2. ease of dispersability in coating solutions PA1 3. stability of matte to manufacturing and processing conditions to control agglomeration, swelling, "squashing", and suspension in coating solutions. PA1 (a) a support, PA1 (b) a thermally processable imaging layer on one side of the support; and PA1 (c) a protective layer comprising a binder and matte particles comprising a crosslinked polymer, wherein the protective layer has been applied as a solution of binder and matte particles in a coating solvent in which the binder is soluble and the matte particles are swellable to the extent of about 160 to about 390%.
The use of limited coalescence made mattes as described in U.S. Pat. No. 5,750,378 has greatly improved particle size distribution and has resulted in a decrease of the over-size population of the as-made matte. This property allows us to use mattes without additional classification to remove the unwanted larger sized particles which in the case of films that use magnification of the final product could give unacceptable visual appearance and/or obscure data of the final product.
The use of methyl methacrylate and other high Tg polymers with and without cross-linking provides a matte that does not change in dimensions in systems when the matte is exposed to high processing temperatures, i.e. near the Tg of the support.